Add ability to sort groups tabs in stand:sidebar

[davibe]

Since the sidebar allows to easily open and recognize so many tabs often you have closer (contiguous) tabs for one topic and then a next group of contigous tabs for another. In this case, even if we don't have advanced grouping functionality like Firefox it would still be nice to be able to select and move a group of contiguous tabs. i.e. "Move the tabs from 2 to 5 to after the one in position ten" -> click the 2nd, hold shift, click the 5th, release shift, drag group after 10th. I think this behaviour is already supported by the list it just has to be enabled.